Sir Joseph John Thomson (18 December 1856 - 30 August 1940), often known as "JJ", was an English physicist, the discoverer of the electron.
Thomson was born in 1856 near Manchester, England, of Scottish parentage. He studied engineering at Owen's College, Manchester, and moved on to Trinity College, Cambridge. In 1884 he became Cavendish Professor of Physics. In 1890 he married Rose Paget, and he had two children with her. One of his students was Ernest Rutherford, who would later succeed him in the post.
Influenced by the work of James Clerk Maxwell, and the discovery of the X-ray, he deduced that cathode rays (see cathode ray tube) existed of negatively charged particles, which he called "corpuscles", and which are now known as electrons. The electron had been posited earlier, by G. Johnstone Stoney, as a unit of charge in electrochemistry, but Thompson realised that it was also a subatomic particle, the first one to be discovered. His discovery was made known in 1897, and caused a sensation in scientific circles, eventually resulting in his being awarded a Nobel prize (1906).
Prior to the outbreak of World War I, he made another ground-breaking discovery: the isotope. In 1918, he became Master of Trinity College, Cambridge, where he remained till his dead. He died in 1940 and is buried in Westminster Abbey, close to Isaac Newton.